Big earth moving and similar transportation crawler vehicles, as well as certain military vehicles are often provided with a big prime mover and a very complicated multi purpose transmission and hydraulic system. This suffers considerable losses and requires an extensive and power consuming cooling system. Alternatively a more complicated engine plant including two prime movers is used, one prime mover being used for propelling the vehicle, and the other (smaller) prime mover being used for driving auxiliary components.
When two prime movers of about the same size are included, both are usually utilized for propulsion purposes, and one of them furthermore for driving bigger auxiliaries and servo systems.
It is evident that such systems will have high transmission losses, are expensive, and on many occasions also demand considerable space and require complicated governing means. The two prime movers of a plant will seldom be operated at full power output simultaneously, as it is evident that a loading apparatus cannot be fully utilized when the vehicle is moving, and reversely the vehicle will not be moved to any extent when the loading apparatus is used.
Tests of a plant including a prime mover and hydraulic transmissions for various purposes have shown some improvements, but the plant still has a number of disadvantages concerning efficiency, cooling, operation in cold climate, weight and servicing.